An image posted on Twitter shows police and other emergency personnel outside a federal building at 201 Varick St. in Manhattan after a gunman killed a security guard and then himself on Friday, Aug. 21, 2015. Photo Credit: Twitter / Shane Kennedy

A gunman shot and killed a security guard at a federal building in Lower Manhattan on Friday before killing himself, and investigators are seeking a motive, police said.

The suspect entered the building just after 5 p.m. and immediately shot the guard with a handgun, police said.

He then walked further into the building toward an elevator when he encountered another employee, said James O'Neill, chief of department for the New York Police Department. The gunman then shot himself in the head.

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Police identified the gunman as Kevin Downing, a 68-year-old former federal employee from Fort Lee, New Jersey. The New York Daily News said he was a retired Army Reserve captain outraged over being fired by the Department of Labor.

Police said the motive of the shooting was unknown, and the investigation was in its earliest stages.

"At this point there is no indication of a nexus to terrorism," O'Neill said. The building houses an immigration court and a U.S. Veterans Health Administration services office.

The guard, Idrissa Camara, 53, of New York, worked for FJC Security Services, which holds a contract with the Department of Homeland Security, said FJC spokesman Michael McKeon.